Pentafluoroethyl iodide is a valuable synthon for introducing the pentafluoroethyl group into organic compounds which, for example, have herbicidal action. However, pentafluoroethyl iodide has achieved greatest significance as a telogen in socalled telomerization with tetrafluoroethylene and/or hexafluoropropene. The perfluoroalkyl iodides which form in the process are important starting materials for numerous syntheses, which give active ingredients having strongly marked hydrophobic and oleophobic properties.
Pentafluoroethyl iodide is prepared predominantly in accordance with equation (I) below from tetrafluoroethylene, iodine pentafluoride and iodine:
5CF2=CF2+IF5+2I2xe2x86x925CF3CF2Ixe2x80x83xe2x80x83(I)
A corresponding process is described, for example, in U.S. Pat. No. 3,406,214. It involved reacting a mixture of iodine pentafluoride and 10% by weight of iodine with gaseous tetrafluoroethylene at from 60 to 80xc2x0 C. A conversion of 40% by weight and a yield of 90% by weight of pentafluoroethyl iodide were achieved.
There has been no lack of attempts to accelerate the preparation process according to the above equation (I) using catalysts, in particular using catalysts from the group of Lewis acid metal compounds, for example TiCl4, ZrCl4 or VF5 (see DE-C-20 33 755). However, a disadvantage of this and similar processes is the increased rate of corrosion on the stainless steel apparatuses which are normally used.
In view of the potential hazard which originates in particular from the extremely reactive and toxic iodine pentafluoride, a preparation plant for pentafluoroethyl iodide by the process according to the equation (I) must satisfy particular requirements with regard to freedom from leaks and must have as few moving parts as possible. The addition of the iodine in liquefied form (by melting under pressure) has proven not to be recommended because of the corrosivity toward metallic materials; even nickel-based alloys are not completely durable during the storage of liquid iodine. Moreover, the exact metered addition of the pressurized, liquid, about 125xc2x0 C.-hot iodine to the iodine pentafluoride, which is present in the reactor and has a temperature of about 90xc2x0 C., has proven to be problematical.
There was therefore a great need for a safe and simple process for the preparation of pentafluoroethyl iodide according to equation (I). The object according to the invention is achieved. essentially by continuous dissolution of solid iodine in iodine pentafluoride. The flow diagram of the process is shown in the Figure. In detail, the process consists of the following steps: A vertical bubble column 1, which is equipped with a level regulator, is filled with IF5 and heated to from 85 to 95xc2x0 C., preferably 90xc2x0 C. A defined amount of crystalline iodine is transferred from an iodine transportation container using a lifting and tilting device 4 into the charge transfer tube 5 and from there is passed batchwise to the iodine dissolution vessel 3 filled with IF5. xe2x80x9cWeakenedxe2x80x9d IF5 is continuously passed into the iodine dissolution vessel 3 from the bubble column 1 using the circulation pump 2. The iodine dissolution vessel 3 is designed such that the filling with solid iodine and the introduction of the xe2x80x9cweakenedxe2x80x9d IF5 is separated from the overflow of the IF5 concentrated with iodine by a calming zone. The IF5 concentrated with iodine to the solution equilibrium is allowed to flow continuously via an overflow into the reactor 1, while tetrafluoroethylene is simultaneously introduced at the foot of the reactor 1 at the rate at which it is consumed by the reaction. Pure pentafluoroethyl iodide escapes at the top of the reactor 1, is liquefied in a cooled condenser and collected in a storage tank. The IF5 consumed in the reaction is replenished, by means of a level regulator in the bubble column 1, from the storage container 6 via the pump 2 into the IF5 cycle. The amount of solid iodine in the dissolution vessel 3 is determined by means of a radioactive level measurement. The solubility of iodine in iodine pentafluoride is 5.2% by weight at 20xc2x0 C. and from 9 to 10% by weight at from 85 to 95xc2x0 C.